(1) Field of the Invention
The present invention relates to an optical transmission system, and more particularly, to an optical transmission system for branching optical signals to allow the optical signals to be communicated among at least three stations or more.
(2) Description of the Related Art
In recent years, demand for international communications is rapidly increasing because of the globalization of business, the spread of the Internet, etc. Under the circumstances, submarine optical transmission systems are an important means of communication comparable to satellite communications, and expeditious realization of economical and large-capacity submarine optical transmission systems are in pressing need.
A submarine optical transmission system is a system whereby stations are interconnected by optical fiber cables laid under water for optical transmission. In the case of a system interconnecting at least three stations or more, an optical branching device is placed between stations to branch optical signals.
For submarine optical transmission on the scale of transoceanic transmission, repeaters are arranged in the middle of optical fiber cables. The repeaters are fed with electricity from stations to repeat and amplify optical signals. The optical branching device also plays the role of switching paths for feeding electricity to such repeaters.
FIGS. 6 and 7 illustrate the configuration of a conventional submarine optical transmission system. The submarine optical transmission system 50 comprises terminal stations 51 and 52, a branch station 53, and an optical branching device 54. The terminal stations 51 and 52 and the branch station 53 are land stations while the optical branching device 54 is placed under water.
In the submarine optical transmission system, lines interconnecting stations include an optical fiber cable for transmitting optical signals and a metallic power supply line for feeding electricity. Accordingly, the configuration of optical fiber cables and the configuration of power supply lines are separately illustrated in FIGS. 6 and 7, respectively.
Referring to FIG. 6, where the terminal stations 51 and 52 are stations operated in different countries, for example, the transmission distance is significantly long. In such cases, repeaters 61 to 64 are inserted in lines L1 and L2 interconnecting the terminal station 51 and the optical branching device 54 (lines interconnecting a terminal station and an optical branching device are referred to as trunk lines), and repeaters 65 to 68 are inserted in trunk lines L3 and L4 interconnecting the terminal station 52 and the optical branching device 54.
The repeaters 61 to 68 of the trunk lines have repeater amplifiers 61a to 68a, respectively, for amplifying optical signals flowing from the terminal station 51 toward the terminal station 52, and also have repeater amplifiers 61b to 68b, respectively, for amplifying optical signals flowing from the terminal station 52 toward the terminal station 51.
In the case of inserting repeaters in trunk lines, provided the interval (span length) between repeaters is X, the repeaters 62, 64, 65 and 67 which are nearest to the optical branching device 54 are generally located at a distance of about ½ of the span length X from the optical branching device 54. Alternatively, such repeaters are arranged at locations such that the distance of the sum of the two intervals (e.g., {distance between the repeater 62 and the optical branching device 54}+{distance between the repeater 65 and the optical branching device 54}) is nearly equal to the span length X.
On the other hand, repeaters 71 and 72 are inserted in lines (referred to as branch lines) L5 and L6 interconnecting the branch station 53 and the optical branching device 54. The branch station 53 is situated, for example, on an island located in the ocean between the terminal stations 51 and 52.
The distance between the branch station 53 and the optical branching device 54 is shorter than the transmission distance between each terminal station and the optical branching device and therefore, in some cases, no repeaters are needed. Usually, however, repeaters are inserted also in the branch lines because of the need to extend the cable length by reason of geographical features of the ocean floor etc. or to improve the quality of optical transmission (generally, where the distance Y between the branch station and the optical branching device is longer than about ½ of the span length X, repeaters are inserted also in the branch lines).
The repeaters 71 and 72 of the branch lines have repeater amplifiers 71a and 72a, respectively, for amplifying optical signals transmitted from the branch station 53 to the optical branching device 54, and also have repeater amplifiers 71b and 72b, respectively, for amplifying optical signals transmitted from the optical branching device 54 to the branch station 53.
To briefly explain the flow of optical signals, an optical signal output from the terminal station 51 onto the trunk line L1, for example, is amplified by the repeater amplifiers 61a and 62a and received by the optical branching device 54. The optical signal is output through the optical branching device 54, then amplified by the repeater amplifiers 65a and 66a of the trunk line L3, and received by the terminal station 52. Also, an optical signal output from the terminal station 51 onto the trunk line L2 is amplified by the repeater amplifiers 63a and 64a and received by the optical branching device 54. Then, the optical signal is diverted toward the branch station 53 by the optical branching device 54, amplified by the repeater amplifier 71b of the branch line L5, and received by the branch station 53.
FIG. 7 illustrates the system configuration in terms of power supply lines. The terminal stations 51 and 52 have power supply devices 51a and 52a, respectively, and the branch station 53 has a power supply device 53a. FIG. 7 shows only power supply lines Ls1, Ls3 and Ls5 associated with the trunk lines L1 and L3 and the branch line L5, respectively, and their related component parts.
The repeater 61 includes a power supply section 61a–s for feeding electricity to the repeater amplifier 61a and a power supply section 61b–s for feeding electricity to the repeater amplifier 61b. Similarly, the repeaters 62, 65, 66 and 71 include power supply sections 62a–s, 65a–s, 66a–s and 71a–s for feeding electricity to the repeater amplifiers 62a, 65a, 66a and 71a, respectively, and power supply sections 62b–s, 65b–s, 66b–s and 71b–s for feeding electricity to the repeater amplifiers 62b, 65b, 66b and 71b, respectively.
The optical branching device 54 has switches SW1 to SW4 for switching power feeding paths. The switches SW1 to SW4 have terminals connected in such a manner that the terminals a and e, the terminals b and c, and the terminals d and f are respectively connected to each other by a fixed line. The terminal h of the switch SW4 is grounded.
In the illustrated state, the switch SW1 is switched to the terminal a side, the switch SW2 is open, and the switch SW3 is switched to the terminal e side. Accordingly, the power supply lines Ls1 and Ls3 are connected and electric current flows in the direction from the power supply device 51a (+) to the power supply device 52a (−), so that electricity is fed to the repeaters 61, 62, 65 and 66.
Also, the switch SW4 is switched to the terminal h side. Thus, current flows through the power supply line Ls5 in the direction from the power supply device 53a (+) to the ground (GND), so that electricity is fed to the repeater 71. While in this state, all repeaters on the lines are capable of operation, permitting optical communication among the terminal stations 51 and 52 and the branch station 53.
FIG. 8 illustrates a switched state of the power feeding paths in the case where a line fault has occurred. If a line fault occurs at the location shown in FIG. 8, communication among all stations is interrupted. It is therefore necessary that the switches SW1 to SW4 of the optical branching device 54 be switched so as to continue communication service within an as broad range as possible.
In the switched state shown in FIG. 8, the switch SW1 is switched to the terminal b side, the switch SW2 is switched to the terminal c side, and the switch SW4 is switched to the terminal g side. Also, the power supply device 53a in the branch station 53 is changed to negative power supply.
Accordingly, the power supply lines Ls1 and Ls5 are connected and current flows in the direction from the power supply device 51a (+) to the power supply device 53a (−), whereby electricity is fed to the repeaters 61, 62 and 71, permitting communication to be continued between the terminal station 51 and the branch station 53.
Meanwhile, as conventional techniques relating to the optical amplification function applicable to repeaters etc., techniques have been proposed in which pump light is introduced into a rare earth-doped fiber and the residual pump light is reflected and is made to again enter the rare earth-doped fiber for the purpose of optical amplification (e.g., Unexamined Japanese Patent Publication No. H09-179152 (paragraph nos. [0072] to [0090], FIG. 1); and Unexamined Japanese Patent Publication No. 2001-117126 (paragraph nos. [0046] to [0048], FIG. 1)).
In the submarine optical transmission system 50 explained above, the repeaters 71 and 72 are inserted in the branch lines L5 and L6, respectively, and thus need to be fed with electricity from the branch station 53. Also, the optical branching device 54 is required to perform switching control for the power feeding paths in case a line fault occurs, in order to continue communication service within an as broad range as possible, and thus is constantly put under high pressure. Accordingly, the optical branching device 54 needs to have a high pressure-resistant structure and requires high pressure-resistant electric relays (vacuum relays etc.), and this makes the device expensive. Also, since the branch station 53 includes the power supply device 53a, the cost of the overall system increases.
If the repeaters 71 and 72 can be omitted from the branch lines L5 and L6, then the optical branching device 54 need not have the power switching function and the branch station 53 need not be equipped with the power supply device 53a, making it possible to construct an inexpensive system.
However, since the branch station 53 and the optical branching device 54 are interconnected with no repeater amplifiers arranged therebetween, the optical transmitting/receiving function of the branch station 53 is required to meet rigorous specifications. To avoid this, the optical branching device 54 may be located as near to the branch station 53 as possible to shorten the transmission distance. Because of the problem of geographical features of the ocean floor or cable route, however, it is very often difficult in practice to locate the optical branching device 54 near the branch station 53. For this reason, it has been difficult up to the present to construct economical submarine optical transmission systems.
According to the aforementioned conventional techniques (Unexamined Japanese Patent Publications No. H09-179152 and No. 2001-117126), the pump light is introduced into a rare earth-doped fiber and the residual pump light is also used to amplify the optical signal input to the device. However, these techniques are focused only on efficient use of the pump light and no consideration is given to construction of systems requiring no repeaters in the branch lines.